In recent years, various processes for producing cans and other containers have been developed. Especially, a process for producing cans and containers by seamwelding a steel strip by means of an electric resistance welding method; for example, using a Soudronic Welder, has been remarkably developed.
In order to use a steel strip for producing cans and containers, it is necessary that the steel strip exhibits an excellent workability and weldability and a satisfactory resistance to corrosion, and resistance to corrosion under lacquer coating.
In the conventional processes, the welded cans and containers are produced from a tin-plated steel strip, that is, so called tin plate, or a hydrated chromium oxide-coated steel strip which is prepared by electrolytically treating a steel strip substrate with chromic acid. The latter type of hydrated chromium oxide-coated steel strip is so-called TFS strip (tin-free steel strip).
However, the above-mentioned tin-plated steel strip exhibits the following disadvantages in the welded can or container-producing process using the electric resistance welding method.
1. It is expected that the use of the tin-plated steel strip causes the cost of producing the welded can or container to the reduced. However, since the tin-plated steel strip is not so economical, the reduction in the canor container-producing cost is unsatisfactory.
2. Also, it is expected that the tin-plated steel strip causes the appearance of the seam portion of the welded can or container to be satisfactory because, the appearance thereof is symmetrical. However, since the welding procedure applied to the tin-plated steel strip causes an undesirable iron-tin alloy layer to be produced in the heat-affected zone in the weld and the surface of the plated tin layer is remarkably oxidized, the surface of the plated tin layer is discolored and the lacquer-bonding property of the plated tin layer is degraded.
The coating layer in the TFS strip consists of metallic chromium and hydrated chromium oxides. Also, it is known that the TFS strip can be produced at a relatively low cost. However, the metallic chromium and the hydrated chromium oxides in the coating layer cause the weldability of the TFS strip to be poor. When a can is produced by welding the TFS strip, the welding strength of the weld seam portion is unsatisfactory. Also, in the welding procedure, a portion of the coated chromium and hydrated chromium oxides in the weld portion is scattered so as to stain not only the weld portion, but also the remaining portion of the can. This phenomenon results in a stained appearance on the entire surface of the can.
In order to eliminate the above-mentioned disadvantages of the TFS strip, it is necessary to mechanically remove a portion of the coating layer on the weld by means of, for example, grinding. However, since the coating layer of the TFS strip is usually composed essentially of from 70 to 150 mg of a metallic chromium underlayer per m.sup.2 of each steel strip substrate surface and 10 to 30 mg of a hydrated chromium oxide upperlayer per m.sup.2 of each steel strip substrate surface, it is difficult to remove the portion of the coating layer by mechanical means, for example, grinding. Also, this grinding operation applied to the TFS strip causes the coating layer to be divided into fine particles and a portion of the fine particles of one remains in the seam portion of the TFS strip to be welded so as to stain the weld on the can. Accordingly, it is difficult to obtain a welded can or container with the weld portion having a satisfactory appearance, from the conventional tin-plated steel strip or the TFS strip.
Japanese Patent Application Publication Nos. 36-15252(1961) and 36-10064(1961) disclose a coated steel strip which is plated with nickel and coated with hydrated chromium oxides and which is usable for producing containers by means of soldering.
In view of the disclosure of the Japanese patent application publications, it is clear that the coated steel strip is inadequate for the electric resistance welding procedure and, therefore, usable only for the soldering procedure.
Belgian patent No. 865,187 discloses a process for producing a coated steel strip usable for producing containers by means of electric resistance welding, in which process, a plated tin layer is formed on a surface of a steel strip substrate by means of an electrical plating, and then, heated so as to form an iron-tin alloy layer, and the tin layer surface is coated with a hydrated chromium oxide layer. However, the iron-tin alloy layer makes the formation of continuous, uniform nuggets to be difficult and causes the weldability of the coated steel strip to be poor. Also, the welding procedure applied to the coated steel strip causes the appearance of the lacquered steel strip to be poor.
Under the above-mentioned circumstances, it is strongly desired by the can- and container-producing industry to provide a hydrated chromium oxide-coated steel strip capable of being firmly welded without mechanically removing the coating layer from the portion thereof to be welded, and capable of obtaining a non-stained surface appearance of the welded portion thereof.